The origins, patterns of differentiation and relative numbers of the three main cell types in bone - osteoblasts, osteocytes, and osteoclasts - will be studied in relation to osteogenesis in the embryo and subsequent skeletal development and maturation using the techniques of stereology applied at both the light and electron microscope levels. The aim will be to develop better understanding of the ways in which the relative abundance of different cell types is involved in the development and function of skeletal structures, and the changes which occur in the number and size of different types of subcellular organelles in relation to the development of specialized functions such as collagen biosynthesis and calcium transport in specific cell types. The ways in which these phenomena may be influenced by hormones such as parathyroid hormone, calcitonin, sex hormones, and Vitamin D will be examined when the normal patterns have been determined.